Irradiation cross-linkable thermostable polymer system, for microelectronic applications

ABSTRACT

Linear fluorooligomers having at least two reactive end groups per polymer molecule are incorporated into radiation sensitive polymer systems which have improved continuous temperature resistance and low dielectric constant. The polymer systems can be applied as lacquers. Preferably perfluorated poly-ethers and perfluorated alkanes are used as starting compounds. The polymeric product is usable as a coating for the production of printed multi-layer wirings and economises on through-bores and additional copper intermediate layers. A further field of application exists in the field of integrated semiconductor circuits in VLSI-technology for the production of negative photo-resists.

This is a division of application Ser. No. 762,513, filed Aug. 5, 1985,now U.S. Pat. No. 4,732,843.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention lies in the field of thermostable polymer systems whichcan be cross-linked by irradiation and which are adapted formicroelectronic applications.

2. Prior Art

It is known that multi-layered wiring laminates can be produced bypressing onto one another thin layers of printed circuits which eachcontain an appropriate wiring image using adhesive foils betweenadjacent layers. The printed circuit layers and the adhesive foils forminsulating layers between individual conductive paths. Following theformation of the multi-layered laminated circuit boards, the wiringconnections and contact points between the individual layers areproduced by means of bores and subsequent through-contacting of thesebores.

In order to avoid crack formations in the adhesive contact layer and inthe intermediate printed circuit layers, and delamination effects causedduring soldering or in temperature stress changes, the process asdescribed above has been improved upon by providing that both sides of athin-layer printed circuit are provided with a copper layer, and eachsuch copper layer is overcoated with a photo-resist coating. Thisphotoresist coating, which can represent a negative or a positive imagewhen exposed (imaged) by light while covered with a corresponding mask,is developed after imaging. The change effectd by light exposure isusually one of solubility and results in solvent discrimination betweenexposed and unexposed areas. Photo cross-linking and photoinitiatedpolymerization decrease solubility, whereas photomodification offunctionality and photodegradation increase it. The remaining developedresin in regions in a developed residual layer serves as resist regionsduring a subsequent etching process for the copper. A desired conductiveimage is thus formed in areas to which an imaged and developed resistcoating adheres when a negative resist coating is used and after theexposed copper has been removed by etching. These resin residues from animaged and developed photo resist coating are subsequently removedeither with organic solvents or mechanically, and then a next layer isapplied.

In the case of image formation by means of irradiation, it is necessaryto differentiate between a low energy radiation region (wavelengthexceeding about 100 nm) and a high energy radiation region, for example,X-rays or electron rays. Naturally, the image resolution from anexposure to an imaging radiation source increases when radiation of ashorter wavelength is used. As disclosed in an article by A Ledwith in"IEEE Proceedings", Vol. 130, Part 1, No. 5, October 1983 on pages 245to 251, the resolution limits are about 1 μm for ultraviolet (UV)radiation and about 80 Å for electron radiation.

Conductive path intervals (widths) of less than about 20 μm are requiredfor the construction of microelectronic components. Consequently, thematerial which is used must have a low dielectric constant. As disclosedin an article by A. J. Blodgett in "Spektrum der Wissenschaft",September, 1983, pages 94 to 106, the dielectric constant needs to havea value of less than about 3. It is disclosed in the same article thatin the case of highly integrated components, during operation, a highthermal continuous stress (load) occurs which is approximately in theregion of about 100° C.

There is a need in the art for new and improved radiation-sensitivecross-linkable polymer systems thermostable which are adapted formicroelectronic applications.

BRIEF SUMMARY OF THE INVENTION

More particularly, the present invention is directed to a new and veryuseful thermostable polymer system which can be cross-linked byirradiation and which is adapted for microelectronic applications.

A principle aim of the invention is to provide a polymer system whichcan be cross-linked by exposure to light and which also possesses thefollowing properties:

1. A dielectric constant less than about 3,

2. A continuous temperature resistance (thermal load stability) greaterthan about 100° C.,

3. A short exposure (imaging) time which is preferably shorter thanabout 5 minutes at an intensity of 100 MW/cm² (for UV-curing), and

4. Suitability for use in multi-layered wiring constructions withoutintermediate copper layers.

It is also desirable to provide a polymer system which can be thermallycross-linked; for example, by exposure to temperatures ranging fromabout 40° to 200° C.

A polymer system which has a combination of the above indicated highresolution limits and good structural stability, and which also has therequired thermal load stability at 100° C. which also can be easilyprocessed using conventional photolithographic processes, has notpreviously been known.

An article by C. D. Eisenbach in "Angewandte Makromolekulare Chemie"109/110 (1982) on pages 101 to 172 discloses polyimide systems which doin fact exhibit good thermal properties following hardenability, but,during hardening, such systems suffer about a 40% loss in mass, and,therefore, exhibit a strong and undesirable shrinkage.

Furthermore, the same article discloses oligoquinoline systems whichhave very good electrical properties, but which are insensitive duringlight-exposure and which also demonstrate self-coloring.

A radiation-sensitive synthetic resin layer on a cinnamic acidepichlorohydrine-bisphenol A-base, capable of partly fulfilling theabove indicated aims on which the present invention is based, has beenprovided in the German patent application No. P 34 24 119.1 and in U.S.patent application Ser. No. 749,588. Good structural resolution andsuitable electrical properties are achieved by means of this polymersystem; however, the thermal load stability is not optimal.

A new and very useful polymer system which fulfills all theabove-described properties is provided by the present invention. Thisnew polymer system is characterized by containing as starting materialslinear fluorooligomers which have at least two reactive end groups permolecule and which have been reacted (converted or condensed) withradiation-sensitive (radiation-responsive) substances. It lies withinthe scope of the invention that such starting oligomers preferablycomprise perfluorinated compounds. Such fluorinated compounds can thenbe reacted or converted into a polymer which is light sensitive andwhich contains the indicated the linear fluorooligomeric groups andwhich is adapted for radiation (including photo) imaging in a briefexposure period. Imaging by exposure to light radiation yields across-linked thermostable polymer, which, after subsequent development,leaves a film residue in exposed areas. Otherwise, if desired, it ispossible to achieve a limited cross-linking of the polymer by shortexposure to irradiation which after developing can be radiation cured.

Methods for making and using such new polymer system are additionallyprovided.

Other and further aims, objects, features, purposes, advantages, uses,and the like for the present invention will be apparent to those skilledin the art from the present specification taken together with thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is an enlarged diagrammatic fragmentary vertical cross sectionalview through a laminate of copper foil and an adhering layer of aninitially photo sensitive and cross-linkable polymeric composition whichin the embodiment shown has already been imaged and developed in accordwith the teachings of this invention; and

FIG. 2 is a view similar to FIG. 1 but with an additional layer of sucha polymeric composition applied over the initial layer, such additionallayer having been processed similarly to the layer of FIG. 1.

DETAILED DESCRIPTION

One preferred class of such starting perfluorinated compounds comprisesperfluorinated ether compounds which are characterized by the chemicalformulae:

    (1) X--CF.sub.2 O--(C.sub.2 F.sub.4 O).sub.m --(CH.sub.2 O).sub.n --CF.sub.2 --Y

and/or

    (2) Z--CF.sub.2 --(C.sub.2 F.sub.4 O).sub.m --(CH.sub.2 O).sub.n --CF.sub.2 --Z

where:

X and Y are each an independently chosen terminal

radical selected from the group consisting of --CH₂ OH, --COOH, --COCl,and --NCO,

Z is a terminal isocyanate group containing moiety of the formula:##STR1##

m and n are each a positive whole number which is greater than 2, andpreferably m and n each range from about 5 to 20.

The starting compounds of formulas (1) and (2) are commerciallyavailable from the Montedison Company.

Another preferred class of such starting perfluorinated compoundcomprises perfluorinated alkanes which are characterized by the chemicalformula:

    (3) X--(CF.sub.2).sub.n --X,

where:

X is a terminal radical selected from the group consisting of (--CH₂ OH)and iodide (--I), and

n is a positive whole number greater than 2, and preferably n rangesfrom about 3 to 25.

The starting compounds of formula (3) are commercially available fromthe Hoechst Company.

Such fluorinated oligomer compounds can be directly converted to linearfluorooligomeric polymeric products which can be cross-linked byradiation. Thus, for example, such fluorinated oligomer compounds aredirectly reactable with radiation sensitive substances, such as at leastone compound selected from the group consisting of cinnamic acid,acrylic acid, methacrylic acid, and their corresponding acid chloridesand other radiation-responsive reactable derivatives thereof. Forinstance, reaction products with Furanacryloyl chloride can be cured byUV-radiation.

For another example, reaction products such fluorinated oligomercompounds with difunctional carboxylic acids each containing at leastone ethylenic double bond, such as maleic acid, or their correspondinganhydrides, such as maleic acid anhydride, can be cured by X-rayradiation.

In order to increase the size of the radiation sensitive (radiationcross-linkable) fluorine containing molecules, and in order to promote across-linking which increases the desired thermal load stability of afluorine containing polymer system of this invention, in a furtherdevelopment of this invention, it is provided that the above indicatedfluorinated starting materials are reacted with other substances toproduce products which still contain a minimum of two reactive endgroups per molecule. For example, --COCl groups can first be linked to(reacted with) at least one multi-functional, non-radiation responsive(sensitive) substance, and, then, in a next step, the resulting fluorinecontaining molecules are reacted with a radiation-sensitive substance(including mixtures thereof).

Examples of suitable such multi-functional, nonradiation sensitivesubstances with which such fluorinated starting compounds can be reactedinclude: (1) poly-functional alcohols (polyols), such as glycerine orpentaerythritol; (2) polyfunctional phenols, such as (a) compoundscharacterized by the chemical formula: ##STR2## where:

Y is selected from the group consisting of ##STR3## and --COOH, and

R is selected from the group consisting of --H and alkyl radicals;

(b) or compounds characterized by the chemical formula: ##STR4## where:

X is --OH,

Y is ##STR5## and

R is selected from the group consisting of --H and alkyl radicals;

(c) or compounds characterized by the chemical formula: ##STR6## where:Z is --CH₂ OH; or (3) bisphenol --A derivatives such as thosecharacterized by the chemical formula: ##STR7## where:

    Z is --CH.sub.2 OH; or

(4) polymers which contain at least one epoxide group per molecule, andat least one radical per polymer repeating unit which is selected fromthe group consisting of hydroxyl (--OH) and ##STR8## where: R isselected from the group consisting of --H and alkyl radicals; and thelike.

For example, one class of such epoxide group containing multi-functionalnon-radiation sensitive polymers suitable for use in the practice ofthis invention is characterized by the chemical formula: ##STR9## where:

A is a divalent radical, preferably ##STR10##

X is selected from the group consisting of --OH and ##STR11##

R is selected from the group consisting of --H and alkyl radicals,

B is selected from the group consisting of --H, alkyl radicals, phenyl,and (preferably) ##STR12## and

n is a positive whole number of at least about 2, and preferably nranges from about 10 to 30.

Another method for making a desired polymer which can bephoto-cross-linked comprises first condensing (reacting) an oligomericfluorinated starting compound with at least one material havingphoto-reactive (light sensitive) groups, and then condensing (reacting)the intermediate product thus formed in a further step with a furthermulti-functional photo reactive substance. The following exemplaryembodiment is given:

In a first step, a fluorinated starting compound of formula (1), (2), or(3) is condensed (reacted) with a compound having photo reactive groups.For example, a compound of formula (3) is reacted with cinnamoylchloride to produce an intermediate identified for convenience asproduct A: ##STR13##

In a second step, product A is then condensed (reacted) with a product Bconsisting of a glyceryl tricinnamate (that is, the reaction product of1 mole of glycerine and 3 moles of cinnamic acid) having the structuralformula: ##STR14## A mole ratio of product A to product B of at leastabout 10:1, for example, produces an end product C having approximatelythe following illustrative chemical formula: ##STR15##

As those skilled in the art will appreciate, the above formula forproduct C is only one of an infinite number of arrangement possibilitiesfor residues of, respectively, product A and product B in thecross-linked product C. Such variations in arrangement possibilitiesoccur because:

(1) All of the following possible combinations can occur: ##STR16##

The product C is 3-dimensionally "infinitely" cross-linked.

(2) An infinite number of stereoisomers can occur. Thus,

for A ##STR17## --CH=CH-X and B ##STR18## --CH=CH--Y a plurality ofstereoisomers exist for each cyclobutane formation (cross-linking):##STR19## As a result of the different combinations under 1), aninfinite number of different conformations are produced for the product.

For the implementation of the reactions, known processes are used toreact epoxides or to achieve condensation reactions, such as thoseprocesses described, for example, by D. Braun in "Praktikum derMakromolekularen Chemie", published by Huthig and Wepf.

A radiation cross-linkabe polymeric product in accordance with theinvention can be formed into a foil and processed by thecompression/lamination procedures, preferably using temperatures in therange from about 40° to 200° C., with or without metallic intermediatelayers, such as those consisting of copper or copper alloys.Alternatively, such a product can be utilized as a solute in varioussuitable solvents for the coating of substrates, consisting inparticular of metal foils or sheets, by lacquering, spraying or dipping.Such a product can be, if desired, formulated further with additiveswhich serve as photo-initiators, sensitizers, or stabilizers. Forexample, a suitable photo-initiator for cinnamates comprises Michlersketone; for acrylates, benzoin derivatives in concentration of about 1to 5 weight percent; and, for example, a suitable stabilizer compriseshydroquinone in concentrations of about 0.1 to 0.5 weight percent.

In the following description relating to the accompanying drawings, theuse of a polymer system in accordance with the invention in order toproduce a multi-layered wiring laminate is described making reference toan exemplary embodiment. As a result of the properties of a polymer ofthis invention system, new laminate structures are obtained utilizing aninsulating carrier that is provided with electrical conductor paths andelectrical through-contacts a polymer system of this invention.

Referring to FIG. 1, there is seen illustratively a copper foil 1 whichcan have a typical thickness ranging from about 10 to 500 μm upon onesurface of which a layer 2 of a photo-cross-linkable insulatingpolymeric material of the present invention is deposited (coated)preferably from a solution so as to provie a dried coated layerthickness ranging typically from about 5 to 20 μm. Such polymer materialcan be compounded with, for example, Michlers ketone as aphoto-initiator. Layer 2 is thus conveniently applied by dipping,spray-lacquering, or the like to produce a (dried) layer thicknessranging from about 5 to 20 μm.

The layer 2 is conveniently but preferably exposed to (imaged by)UV-radiation, and then is developed by solvent treatment or the like insuch manner as to form the desired residual resin portions 13 with openareas 3 therebetween which expose portions of the surface of layer 1.For example, the irradiation is effected by the contact or theprojection process using a mask (not shown) which in effect covers theregion of each of the openings 3 in the layer 2 (negative lacquer). Thecovered parts 3 are then dissolved away using an appropriate solvent,such as, for example, a chlorofluorocarbon (for example, Freon® of theDupont Company or Fluorinert® of the 3M Company). In the case of theresidual exposed portions 13 of the layer 2, a chemical cross-linkinghas taken place during the irradiation which prevents dissolutionthereof so that these parts 13 remain as insulating layer after thedevelopment (solvent treatment).

Following the production of the openings 3 for the through-contactsassigned to this first insulating layer 2 and its residual exposedportions 13, the openings 3 as defined by the portions 13 and foil 1 areeffectively filled in by means of metal plating using an electricallyhighly conductive material, such as for example, copper, therebyproducing the desired through-contact regions 23 as shown in FIG. 2.

Referring to FIG. 2, a further layer 4 is next applied over the layerdefined by portions 13 and regions 23. Layer 4 is likewise comprises ofa photo-cross-linkable insulating polymeric material in accordance withthe invention and is applied in the same way as described in referenceto FIG. 1. In layer 4 conductor paths are formed and then filed withelectrically highly conductive material in the same way as described inreference to FIG. 1, thereby to provide through-contact regions 5 and 6.The exposure and development of the layer 4 likewise takes place asdescribed in FIG. 1. In addition to the openings for thethrough-contact, regions 6 which come into contact with thethrough-contact regions 23 in the first insulating layer portions 13,trench-like recesses for the through contact regions 5 are also producedin the second insulating layer 4, and such trench-like recesses arearranged in such manner that at least one through-contact region 23makes contacting association with through-contact region 5. Thethrough-contact regions 5 and 6 are conveniently formed byelectroplating. Further through-contacts and conductor paths can then beapplied by corresponding repetition of the above-described productionsteps. Between the individual layer formations, the arrangements aresubjectable to a drying and/or thermal curing process by heating totemperatures in the range from about 40° to 150° C.

The novel wiring construction requires no copper intermediate layers.The required high thermal load stability is achieved. The processingwith known photo resist techniques is problem-free. This simplifies notonly the process for the production of such a construction, but also thereliability in respect of electrical characteristics.

For this reason, and also on account of the very low DC and the goodresolution, the polymer systems in accordance with the invention arebest suitable for use as high-temperature-resistant negative resists inthe production of integrated semiconductor circuits in VLSI-technologywhere the production of dimensionally accurate microstructures andpatterns is of great significance. The exposure time is comparable withthe latter systems which can be used in this field. Further details canbe obtained from the article by C. D. Eisenbach in the magazine "DieAngewandte Makromolekulare Chemie" 109/110 (1982) on pages 101 to 112.

As is apparent from the foregoing specification, the invention issusceptible of being embodied with various alterations and modificationswhich may differ particularly from those that have been described in thepreceding specification and desription. For this reason, it is to befully understood that all of the foregoing is intended to be merelyillustrative and is not to be construed or interpreted as beingretrictive or otherwise limiting of the present invention, excepting asit is set forth and defined in the hereto-appended claims.

We claim:
 1. A method for manufacturing a thermostable polymer systemcross-linkable by irradiation which is used for manufacturingmicroelectronic circuits with multilayer wiring comprising the stepsof:(A) reacting an irradiation sensitive acid chloride of at least oneacid selected from the group consisting of cinnamic acid, acrylic acid,and methacrylic acid, with a linear fluorinated oligomer having at leasttwo reactive and groups for the reaction with the acid chloride, and (B)reacting a resulting product with at least one irradiation sensitiveester of said acid and a multifunctional alcohol by irradiation, wherebythe mole ratio of said resulting product to said ester is at least about10:1.